How did you flush your fluid?/
I do mine a couple of ways (not necessarily on an SLK)
Hold down brake pedal, furthest wheel 1st. crack open bleed, close straight away.
Lift pedal slowly.
Repeat again untol fluid color changes .
Do other wheels, ending on closest to MC.
Ensure MC stays over 1/2 full the whole time, top up at finish.
Put tight fitting hose onto bleed nipple, crack open, pump pedal down, slowly raise, do a couple ot times as before,
checking MC etc, tighten bleed, go to next wheel.
Fit pressure bleed cap adaptor to MC, pump a few psi,
crack open bleed to let fluid run out as before, tighten, move to next wheel ensuring MC ful and pressure maintained.
Some times a "FAST" manua; crack of bleed is required to "PUSH" air out of a problem caliper.
Always close bleed BEFORE fluid stops running.